Acoustic wave devices are used for filters and the like of wireless terminals such as mobile phone terminals, for example. Exemplary acoustic wave elements used for an acoustic wave device are a surface acoustic wave element in which an IDT (Interdigital Transducer) electrode is formed on a piezoelectric substrate, and a piezoelectric thin film resonant element in which a piezoelectric film is sandwiched by upper and lower electrodes.
An acoustic wave device using an acoustic wave is required to have a space above a functional part of an acoustic wave element to maintain characteristics of the acoustic wave element. The functional part of the acoustic wave element includes an electrode finger of an IDT electrode in the surface acoustic wave element, and includes a region where upper and lower electrodes sandwiching a piezoelectric film overlap in the piezoelectric thin film resonant element. The above requirement may be met by a structure designed to have a sealing portion having a cavity provided above the functional part of the acoustic wave element. Such a structure is so called a hollow structure.
For example, Japanese Patent Application Publication No. 2004-129193 discloses a technique that defines a sealed space above an IDT electrode by bonding a piezoelectric substrate with a surface acoustic wave element being formed therein to a base substrate. More specifically, after forming a connection electrode for making a connection with the element and a periphery sealing conductor film on the base substrate, a solder bump member (protrusion electrode) is formed on the electrode, and a solder bonding member (seal ring) is formed on the periphery sealing conductor film. The next step is to form the IDT electrode, the connection electrode, and the periphery sealing electrode on the piezoelectric substrate. Then, a sealed space is formed above the IDT electrode by bonding the electrode to the solder bump member and bonding the periphery sealing electrode to the solder bonding member.
In a case where the acoustic wave element is hermetically sealed by bonding the substrate with the acoustic wave element to a mounting substrate, the use of separate processes for forming the protrusion electrode and the seal ring on the substrate increases the number of fabrication steps and the cost. The protrusion electrode has a function of electrically connecting the acoustic wave element to the external, and the seal ring has a function of hermetically sealing the acoustic wave element. In addition, the use of the separate processes for forming the protrusion electrode and the seal ring makes it difficult to equalize the height of the protrusion electrode and the height of the seal ring to each other. Thus, the protrusion electrode may not be electrically connected to the mounting substrate to cause poor electrical characteristics, or the seal ring may not be bonded to the mounting substrate to cause a problem in hermetical seal. Thus, it is preferable that the protrusion electrode and the seal ring are formed simultaneously. When electrolytic plating is used to form the protrusion electrode and the seal ring, simultaneous forming of the protrusion electrode and the seal ring may be achieved by using a plating power-supply line that is electrically unified so as to be connected to both of the protrusion electrode and the seal ring.
However, the use of electrolytic plating may need another process of removing a part of the plating power-supply line to separate the protrusion electrode and the seal ring from each other. The removing step increases the number of fabrication steps and gives an opportunity that a foreign material adheres.